Add lemmas `elem_of_prefix` and `elem_of_suffix`

Merged Jonas Kastberg requested to merge jihgfee/stdpp:elem_of_prefix into master

Added a lemma that captures that any element [x] that is in a prefix [l1] is also in the full list [l2]. The property is only trivial from the definition of l1 prefix_of l2 (i.e. ∃ l, l2 = l1 ++ l2). Adding the lemma thus avoids breaking abstraction.

Edited by Robbert Krebbers

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